1. Field of the Invention
The present invention relates to an isolation structure fabricated on the silicon substrate and more particularly, to a trench isolation structure fabricated on a silicon substrate and also to a method by means of which the trench isolation structure is fabricated.
2. Background of the Related Art
Various isolation structures have been proposed to prevent the establishment of parasitic electrical connections between adjacent devices of integrated circuits fabricated on silicon substrates. Since LOCOS (Local Oxidation of Silicon) is well-characterized, simple to implement, and resistant to contamination, LOCOS processes have been widely employed to fabricate isolation structures. As device geometries reached submicron size, however, conventional LOCOS isolation reached the limit of its effectiveness, since the bird's beak structure characteristic of conventional LOCOS field oxides causes unacceptably large encroachments of the field regions of the substrate into the substrate's active regions and the surface topography of LOCOS field oxides proves inadequate to the planarity requirements of submicron scale lithography.
Advanced LOCOS processes which suppress or entirely eliminate the formation of bird's beak have been used to fabricate isolation structures for both 64-Mbit and 256-Mbit DRAM arrays. However, the field oxides of even advanced LOCOS processes prove too large for utilization in Gbit DRAM arrays, which require memory cells whose gate lengths are shorter than 0.2 .mu.m. Trench isolation structures have been widely employed to overcome the limitations inherent in LOCOS field oxides, primarily because trenches with vertical sidewalls may be made significantly narrower than LOCOS field oxides of the same depth and widths of such trenches are more easily controlled than the widths of LOCOS field oxides. Process steps of a typical method of fabricating a conventional trench isolation structure in a silicon substrate are described immediately below with reference to the idealized cross-sectional views of FIGS. 1A-1D. After a thin pad oxide has been thermally grown on or deposited by chemical vapor deposition (CVD) onto a silicon substrate 10 and a silicon nitride (Si.sub.3 N.sub.4) deposited onto the pad-oxide, the oxide and the nitride are patterned by means of conventional photolithographic and etching process steps to form an oxide film 11 and a nitride film 12, respectively. which together mask active (device) regions of the substrate. (The active regions are, by definition, the regions of the substrate masked by the oxide and nitride films, while the field regions are the regions of the substrate not so masked.) Trenches 13 are then selectively and anisotropically dry-etched in the field (isolation) regions of the substrate 10, as shown in FIG. 1A. As shown in FIG. 1B, a conformal oxide film 14 is deposited onto the nitride film and onto the walls and floor of the trenches 13. The areas of the oxide film 14 which line the trenches 13 serve to repair damage suffered by the substrate when the trenches were etched. As shown in FIG. 1C, a layer of amorphous SiO.sub.2 (known also as fused silica, undoped silica glass, or silica glass) 15 is deposited by CVD onto the conformal oxide film 14 and within each of the trenches 13. Various other dielectrics, including CVD-polysilicons, may be used to refill the trenches. As shown in FIG. 1D, the silica glass layer 15 is etched-back until silica glass remains only within the lined trenches, thus completing fabrication or trench isolation structures 19, each of which the lined trenches, thus completing fabrication of trench isolation structures 19, each of which consists of an oxide trench liner 14a within each of the trenches 13 and a silica glass structure 16a within each of the trench liners 14a.
Although trench isolation structures, particularly those formed in deep, narrow trenches, overcome many of the limitations of LOCOS field oxides, methods of fabricating trench isolation structures, as typified by the method described above, are not without drawbacks. For any trench width, trenches are more likely to be imperfectly formed and incompletely filled as the aspect ratio (i.e., the ratio of depth to width) of the trenches increases. For example, trench sidewall profiles must be strictly controlled to avoid void formation when the trenches are refilled with silica glass or other solid dielectric. Moreover, for any aspect ratio, formation of voids becomes more likely as the trenches become narrower than about 2 .mu.m.